Focused ultrasound (i.e., acoustic waves having a frequency greater than about 20 kilohertz) can be used to image or therapeutically treat internal body tissues within a patient. For example, ultrasonic waves may be used to ablate tumors, eliminating the need for the patient to undergo invasive surgery. For this purpose, a piezo-ceramic transducer is placed externally to the patient, but in close proximity to the tissue to be ablated (“the target”). The transducer converts an electronic drive signal into mechanical vibrations, resulting in the emission of acoustic waves. The transducer may be shaped so that the waves converge in a focal zone. Alternatively or additionally, the transducer may be formed of a plurality of individually driven transducer elements whose phases can each be controlled independently from one another. Such a “phased-array” transducer facilitates steering the focal zone to different locations by adjusting the relative phases between the transducers. Magnetic resonance imaging (MRI) may be used to visualize the patient and target, and thereby to guide the ultrasound beam.
The effectiveness of ultrasound therapy depends on the accuracy of the focus location, the sharpness and shape of the focal zone, and the avoidance of “hot spots” (i.e., regions of high ultrasound intensity) outside the target. Transducer elements that are not properly configured or controlled can lead to improper focus location and reduced focus quality, resulting in less effective therapy, and possibly damage to healthy tissue surrounding the target. It is therefore desirable to correct any mechanical misconfigurations. Improper transducer configuration may result from manufacturing errors, inadvertent shifting of transducer elements from their expected locations during use or repair, deformation of the transducer due to thermal expansion, or a combination of these and other effects. Even slight locational deviations can have significant effects on the quality of the transducer output. For example, as illustrated in FIG. 1, if the height of a curved transducer surface having a width of 120 mm and a nominal radius of curvature of 160 mm changes by only 1 mm, the ultrasound focus shifts by about 13 mm. In a phased array, deviations of the transducer locations from the intended locations can be compensated for by adjusting the phases with which the elements are driven. This procedure is hereinafter referred to as “mapping” the transducer.
One approach to mapping a phased-array transducer surface involves driving each transducer element individually to produce an acoustic wave pulse in water; measuring the arrival of the acoustic wave pulse in three locations using a hydrophone; determining for each location the time of flight, and thus the distance, from the transducer element; and calculating the coordinates of the element location by triangulation from the three measurements. Based on the intended and the measured actual locations of the transducer elements, the necessary phase adjustments can be calculated. This method is described in U.S. Pat. No. 7,535,794 to Prus et al., which is hereby incorporated herein by reference in its entirety. In addition to a hydrophone, implementation of the method requires other auxiliary equipment, such as an amplifier and data-acquisition module. Further, to avoid damaging the hydrophone, the mapping is typically performed at transducer power levels significantly below those used during normal operation, which can undermine the validity of the adjustments under therapeutic conditions. Alternative transducer mapping methods that do not have these drawbacks are therefore desirable.